Advances in technology have resulted in smaller and more powerful computing devices. For example, there currently exist a variety of portable computing devices, including wireless computing devices such as wireless telephones, personal digital assistants (PDAs), portable gaming consoles, and tablet computers that are small, lightweight, and easily carried by users. In order to simplify user interfaces and to avoid pushbuttons and complex menu systems, such portable computing devices typically implement touch screen displays that detect user gestures on the touch screen and translate the detected gestures into commands to be performed by the device. Such gestures may be performed using one or more fingers or a stylus type pointing implement. Multi-touch screens (touch screens having multi-touch capability) are designed to recognize and track several simultaneous touches. For example, when a user moves two fingers on a screen, information indicating touch/movement for both fingers is provided by a multi-touch screen.
There are several touch screen technologies currently available which support multi-touch input, including capacitive, resistive, and optical touch sensing using cameras. Capacitive technology operates by sensing the electric current from a user's finger, which interrupts the electrostatic field of the touch screen, resulting in a detected touch. In some implementations, a touchscreen can include a projected capacitive touch (PCT) sensor arranged over a display. The PCT sensor can include an array of capacitors formed by a number of sensor sensors in the form of overlapping sensors, such as row sensors and column sensors that are arranged in a grid pattern. The high resolution of PCT allows operation with no direct contact, supporting hover touch detection, allowing the panel to be coated with protective insulating layers, and allowing the panel to operate even under screen protectors or behind weather-proof or shatter-resistant panels. Depending on the implementation, an active or passive stylus can be used instead of or in addition to a finger. As such, PCT is a more robust solution versus resistive touch technology. Accordingly, touch sensitive mobile devices commonly implement PCT technology.
Several drawbacks of implementing PCT technology on portable computing relate to the costs typically required for recognizing and locating touch events within the sensor array, leading to undesirable latency and power drainage, particularly because mobile devices generally have more limited processing resources and operate on battery-supplied power. Despite these limitations, many common mobile applications such as maps, games, email clients, web browsers, etc., are making increasingly complex use of touch recognition. Further, panel scan time and touch processing complexity increase proportional to touch-node capacity, which in turn increases proportional to display size. Therefore, because there is a trend in many portable computing devices toward increasing display size and touch complexity, touch processing is increasingly reducing device performance and threatening battery life. Further, user interaction with a device through touch events is highly sensitive to latency, and user experience can suffer from long scan times for large touchscreen panels, resulting in processing delay and response lag.